1. Field of the Invention
The present invention generally relates to the design of water-borne vessels and simulation of performance by model testing and, more particularly, to towing posts for measurement of forces on models under simulated operating conditions in towing basins.
2. Description of the Prior Art
Hull designs for water-borne vessels including surface ships and submarines is a highly complex process with substantial economic consequences. Given the extended useful lifetime generally characteristic of such vessels, a small loss of performance efficiency, such as a small increase in power required to maintain operation at a desired design speed, may translate into hundreds of thousands of dollars in fuel costs. Further, increased power requirements for certain desired performance may require larger power plants of increased weight which reduce the potential performance of the vessel. The cost of building vessels of even moderate size also dictates that the highest degree of certainty of performance predictions and efficiency must be obtained concerning the design before construction is begun.
Therefore, it has become customary to simulate the performance of vessel designs by model testing in towing basins in which models may be towed at various speeds scaled to desired operating conditions and in which wave action may also be accurately simulated. In such towing basins, movement of the model is achieved by attachment of the model (ideally at its center of pressure) to a carriage which traverses the basin. This attachment is generally designed to ideally allow only a few degrees of freedom to the model, such as roll, pitch and heave, so that the attitude of the vessel may be otherwise maintained in accordance with the desired test conditions. For example, in a drag test, the model should not be allowed to yaw since yaw will greatly increase values of drag for a very small angular displacement (often only a fraction of a degree) of the hull to the direction of motion.
The term "center of pressure", as used in this description of the invention as well as previous towing arrangements, is a location directly above the static center of buoyancy of the hull by a distance known as the metacentric height. That is, The "center of pressure" is at a location about which pitch and roll should occur. This usage is somewhat different from the other usages of the term, particularly in regard to the stability of sailing craft or the dynamic center of pressure of water on a hull due to planing or wave conditions. However, dynamic variation of the instantaneous center of pressure from the location defined above generally results in motions of the hull (e.g. pitch and roll or heaving) which are permitted and can be accounted for in the drag measurement. For example, changes in drag due to attitude (e.g. bow rise) as a planing hull changes from displacement mode to planing mode as speed increases is a measurement of interest in drag tests. Therefore, defining the "center of pressure" as being a metacentric height above the static center of buoyancy does not introduce errors into drag tests and, in some cases, avoids masking of information of interest in drag tests.
Other performance characteristics of the hull design, such as tendency to roll, yaw and pitch, can be evaluated by appropriate instrumentation or observation. Heaving (vertical translation of the center of pressure of the hull), surging (axial variation in speed of the center of pressure) and sway (lateral displacement of the center of pressure) under simulated sea and operational conditions may also be observed and/or measured. However, for the economic reasons mentioned above, drag testing remains one of the most common tests performed and the type of test which requires a great degree of accuracy, particularly with the recent increased interest in development of planing hulls.
While the general effectiveness of model testing in towing basins has been well-accepted for many years, several unavoidable sources of error have been inherent in towing arrangements which have been in use prior to this invention. The errors are particularly deleterious for drag testing but cause errors in other types of testing as well.
Specifically, the towing carriage should not provide a clearance above the water surface which is significantly greater than is required to accommodate heave of the model because the towing post is essentially a cantilever structure which must not deflect under towing loads. Otherwise, the geometry of the test set-up and the orientation of force sensors may be compromised. The towing post must also accommodate heaving of the model while carrying potentially large loads without placing detectable loads on the model. For this reason, rotary bearings have generally been used at either end of a towing arm, one end of which is attached to the model through a gimbal and the other attached to the towing post located forward of the gimbal attachment.
This arrangement inherently compromises the geometry of the test since heaving of the model would cause an angular change of up to twenty degrees in some cases in the coupling provided by the towing arm. In a drag test, this angular change also changes the angle of the force vector being measured and thus modulates the drag force. Further, due to the inability of a towing arm to restrain yaw, it was customary in the past to attach the towing arm to a point near the bow of the model rather than at the center of pressure. Therefore, the drag measurement was also modulated by pitching of the model. Additionally, increase in size of models has also increased the need to move the attachment point closer to the bow.
Any yawing of the model would also modulate the drag force measurement by the same mechanism as well as causing measurement of drag at the yaw angle rather than with the model held in correct trim. By the same token, since the prior towing arm allowed neither restraint of yaw nor measurements free from the effects thereof, adjustment of model yaw could not be accurately achieved. Since exact symmetry of models is not achievable and, in any event, ideal symmetry may not accurately characterize the hull shape of a constructed vessel, correct yaw cannot be established by merely aligning nominal model centerline with towing motion. Other effects may also inherently disturb drag measurements or trim of the model or both.
Planing hulls, in particular, have posed problems which have proved intractable with prior towing arrangements. In particular, planing hulls are operated at high speed and drag forces may be encountered which are far higher than for displacement hulls. More importantly, however, displacement hulls tend to sink slightly with increases in speed since negative lift is developed by the hull bottom. In contrast, planing hulls, by definition, provide positive lift through planing and the vertical movement of the hull due to this lift is many times the amount of vertical displacement usually observed with displacement hulls and accounts for the improvement of efficiency, at planing speeds, of planing hulls.
Yet another effect, although generally small, is that of inertial forces of the towing arm on the model during angular change. As the model pitches or heaves, the inertia due to any motion of towing structure not located at the center of pressure of the model will be reflected in a change of fore-and-aft trim of the model which will also modulate the drag force measured, perhaps in a highly unpredictable fashion.
Thus it is seen that the prior towing arrangements for model testing in towing basins included many inherent sources of error, particularly for critical drag measurements. At the present degree of refinement of hull design and the cost of construction of large vessels and fuel to power such vessels during their useful lifetime, the data provided by towing tests conducted in accordance with prior towing arrangements is of relatively reduced value because of the inaccuracy inherent in the previous towing structures. Despite the inherent inaccuracies, however, no towing arrangement offering more than a marginal accuracy improvement has been proposed.